Re: [RFC PATCH] sched: Add scx_cpuperf_target in sched_cpu_util()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 18, 2026 at 03:08:43PM -1000, Tejun Heo wrote:
> On Wed, Mar 18, 2026 at 01:47:18PM +0100, Peter Zijlstra wrote:
> > On Wed, Mar 18, 2026 at 08:17:55PM +0800, Xuewen Yan wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index bf948db905ed..20adb6fede2a 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -8198,7 +8198,12 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
> > >  
> > >  unsigned long sched_cpu_util(int cpu)
> > >  {
> > > -	return effective_cpu_util(cpu, cpu_util_cfs(cpu), NULL, NULL);
> > > +	unsigned long util = scx_cpuperf_target(cpu);
> > > +
> > > +	if (!scx_switched_all())
> > > +		util += cpu_util_cfs(cpu);
> > > +
> > > +	return effective_cpu_util(cpu, util, NULL, NULL);
> > >  }
> > 
> > This puts the common case of no ext muck into the slow path of that
> > static_branch.
> > 
> > This wants to be something like:
> > 
> > unsigned long sched_cpu_util(int cpu)
> > {
> > 	unsigned long util = cpu_util_cfs(cpu);
> > 
> > 	if (scx_enabled()) {
> > 		unsigned long scx_util = scx_cpuperf_target(cpu);
> > 
> > 		if (!scx_switched_all())
> > 			scx_util += util;
> > 
> > 		util = scx_util;
> > 	}
> > 
> > 	return effective_cpu_util(cpu, util, NULL, NULL);
> > }
> 
> scx_switched_all() is an unlikely static branch just like scx_enabled() and
> scx_cpuperf_target() has scx_enabled() in it too, so the difference for the
> fair path between the two versions is two noop run-throughs vs. one. Either
> way is fine but it is more code for likely no discernible gain.

(added noinline to effective_cpu_util() for clarity)

So the original patch generates this:

sched_cpu_util:
1c5240:  sched_cpu_util+0x0       endbr64
1c5244:  sched_cpu_util+0x4       call   0x1c5249 <__fentry__>
1c5249:  sched_cpu_util+0x9       push   %rbp
1c524a:  sched_cpu_util+0xa       push   %rbx
1c524b:  sched_cpu_util+0xb       mov    %edi,%ebx
1c524d:  sched_cpu_util+0xd       <jump_table.1c524d>
                                  = nop2                                   (if DEFAULT)
                                  = jmp    1c5271 <sched_cpu_util+0x31>    (if JUMP)
1c524f:  sched_cpu_util+0xf       xor    %ebp,%ebp
1c5251:  sched_cpu_util+0x11      <jump_table.1c5251>
                                  = nop2                                   (if DEFAULT)
                                  = jmp    1c5261 <sched_cpu_util+0x21>    (if JUMP)
1c5253:  sched_cpu_util+0x13      xor    %edx,%edx
1c5255:  sched_cpu_util+0x15      xor    %esi,%esi
1c5257:  sched_cpu_util+0x17      mov    %ebx,%edi
1c5259:  sched_cpu_util+0x19      call   0x1bc5b0 <cpu_util.constprop.0>
1c525e:  sched_cpu_util+0x1e      add    %rax,%rbp
1c5261:  sched_cpu_util+0x21      mov    %rbp,%rsi
1c5264:  sched_cpu_util+0x24      mov    %ebx,%edi
1c5266:  sched_cpu_util+0x26      xor    %ecx,%ecx
1c5268:  sched_cpu_util+0x28      pop    %rbx
1c5269:  sched_cpu_util+0x29      xor    %edx,%edx
1c526b:  sched_cpu_util+0x2b      pop    %rbp
1c526c:  sched_cpu_util+0x2c      jmp    0x1c5160 <effective_cpu_util>

(slowpath)

1c5271:  sched_cpu_util+0x31      movslq %edi,%rdx
1c5274:  sched_cpu_util+0x34      mov    $0x0,%rax
1c527b:  sched_cpu_util+0x3b      mov    0x0(,%rdx,8),%rdx
1c5283:  sched_cpu_util+0x43      mov    0xa34(%rdx,%rax,1),%ebp
1c528a:  sched_cpu_util+0x4a      jmp    0x1c5251 <sched_cpu_util+0x11>


While my proposal generates this:

sched_cpu_util:
1c5240:  sched_cpu_util+0x0       endbr64
1c5244:  sched_cpu_util+0x4       call   0x1c5249 <__fentry__>
1c5249:  sched_cpu_util+0x9       push   %rbx
1c524a:  sched_cpu_util+0xa       xor    %esi,%esi
1c524c:  sched_cpu_util+0xc       xor    %edx,%edx
1c524e:  sched_cpu_util+0xe       mov    %edi,%ebx
1c5250:  sched_cpu_util+0x10      call   0x1bc5b0 <cpu_util.constprop.0>
1c5255:  sched_cpu_util+0x15      mov    %rax,%rsi
1c5258:  sched_cpu_util+0x18      <jump_table.1c5258>
                                  = nop2                                   (if DEFAULT)
                                  = jmp    1c5266 <sched_cpu_util+0x26>    (if JUMP)
1c525a:  sched_cpu_util+0x1a      mov    %ebx,%edi
1c525c:  sched_cpu_util+0x1c      xor    %ecx,%ecx
1c525e:  sched_cpu_util+0x1e      xor    %edx,%edx
1c5260:  sched_cpu_util+0x20      pop    %rbx
1c5261:  sched_cpu_util+0x21      jmp    0x1c5160 <effective_cpu_util>

(slowpath)

1c5266:  sched_cpu_util+0x26      <jump_table.1c5266>
                                  = nop2                                   (if DEFAULT)
                                  = jmp    1c527b <sched_cpu_util+0x3b>    (if JUMP)
1c5268:  sched_cpu_util+0x28      xor    %eax,%eax
1c526a:  sched_cpu_util+0x2a      <jump_table.1c526a>
                                  = nop2                                   (if DEFAULT)
                                  = jmp    1c5296 <sched_cpu_util+0x56>    (if JUMP)
1c526c:  sched_cpu_util+0x2c      mov    %ebx,%edi
1c526e:  sched_cpu_util+0x2e      add    %rax,%rsi
1c5271:  sched_cpu_util+0x31      xor    %ecx,%ecx
1c5273:  sched_cpu_util+0x33      xor    %edx,%edx
1c5275:  sched_cpu_util+0x35      pop    %rbx
1c5276:  sched_cpu_util+0x36      jmp    0x1c5160 <effective_cpu_util>
1c527b:  sched_cpu_util+0x3b      movslq %ebx,%rdx
1c527e:  sched_cpu_util+0x3e      mov    $0x0,%rax
1c5285:  sched_cpu_util+0x45      mov    0x0(,%rdx,8),%rdx
1c528d:  sched_cpu_util+0x4d      mov    0xa34(%rdx,%rax,1),%eax
1c5294:  sched_cpu_util+0x54      jmp    0x1c526a <sched_cpu_util+0x2a>
1c5296:  sched_cpu_util+0x56      mov    %ebx,%edi
1c5298:  sched_cpu_util+0x58      mov    %rax,%rsi
1c529b:  sched_cpu_util+0x5b      xor    %ecx,%ecx
1c529d:  sched_cpu_util+0x5d      xor    %edx,%edx
1c529f:  sched_cpu_util+0x5f      pop    %rbx
1c52a0:  sched_cpu_util+0x60      jmp    0x1c5160 <effective_cpu_util>


That fastpath is definitely better; the slowpath is worse, but that is
in part because the compilers are stupid and cannot eliminate
static_branch().

/me goes try again .. Yeah, the below patch does nothing :-( It will
happily emit scx_enabled() twice.


---
diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 05b16299588d..47cd1a1f9784 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -32,7 +32,7 @@
 	JUMP_TABLE_ENTRY(key, label)
 #endif /* CONFIG_HAVE_JUMP_LABEL_HACK */
 
-static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
+static __always_inline __const bool arch_static_branch(struct static_key * const key, const bool branch)
 {
 	asm goto(ARCH_STATIC_BRANCH_ASM("%c0 + %c1", "%l[l_yes]")
 		: :  "i" (key), "i" (branch) : : l_yes);
@@ -42,7 +42,7 @@ static __always_inline bool arch_static_branch(struct static_key * const key, co
 	return true;
 }
 
-static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
+static __always_inline __const bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {
 	asm goto("1:"
 		"jmp %l[l_yes]\n\t"
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index c16d4199bf92..553fc9f3f7eb 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -312,6 +312,7 @@
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
  */
 #define __pure                          __attribute__((__pure__))
+#define __const                         __attribute__((__const__))
 
 /*
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-section-function-attribute